The simple answer is we don’t know yet! Less than 10% of microorganisms can be successfully grown in a lab so there are lots that we know very little about. Of the ones we can grow, the mycoplasmas are the smallest living organisms and they have no cell wall. Using powerful microscopes, scientists are now reporting nanobacteria in the environment, these bacteria are around the same size as the mycoplasmas and they do have a cell wall. As far as I know, no-one has managed to grow the nanobacteria in the lab yet.
If you consider viruses as “living organisms”, some scientists don’t, then I suspect they would be some of the smallest living organisms identified.
There are also self replicating RNA (a molecule similar to, but smaller than, DNA), but unlike viruses it does not seek out cells to reproduce in. Instead it reproduces in the presence of particular enzymes/chemicals. We do not consider these as living organisms.
The answer to this question could be different depending on what you mean by ‘smallest’! Is it smallest volume, smallest weight, smallest genome? The mycoplasma Elise mentioned are about 500 times smaller than the width of a human hair.
Surprisingly, this isn’t a straightforward question! Elise is right – we don’t know for sure yet because we are always finding new organisms and have barely scratched the surface in terms of knowing what is out there. Dirk also raises a great point – that depending on what you define as life, depends on what you can count. But in an effort to try to answer this question, here’s what is classified the smallest so far based on the various things that Renee listed may count as ‘the smallest':
The smallest virus (if you consider viruses ‘living’, and many scientists argue they aren’t as per Dirk’s comments) is a virus called Porcine circovirus type1, its genome is only 1759 bp long.
The smallest organism based genome size is a bacteria called Candidatus Carsonella ruddii. Its genome is only 159,662bp long.
The smallest organisms based on actual physical size, and capable of self-replication are what Elise and Renee said – Mycoplasma genitalium and Nanoarcheum equitans. There are also organisms called nanobes that may also be considered some of the smallest organisms known to science so far.
The smallest vertebrate (organism with a backbone) is a frog from Papua New Guinea – it is only about 7.7mm in size!
And the smallest mammal is the Kitti’s Hog-nosed bat, and it is only about 30mm in length and weighs 1.5 – 2 grams.
Not sure, I guess viruses are quite small, but then again some scientists don’t think they classify as living organisms. Here is the wikipedia page on small organisms if you want further reading http://en.wikipedia.org/wiki/Smallest_organisms
Have you seen the visualisation on how small these small things really are: http://learn.genetics.utah.edu/content/begin/cells/scale/ You go from coffee bean which is 12×8 mm to carbon atom at 340 pm (picometer 10 to the power of minus 12) by using the slider and there are lots of interesting living and non-living things along the way.
Did you know that the tasmanian devil is suffering from contagious cancer? We used to think that cancer could not be transmitted, but the tasmanian devils are managing to spread it to each other. http://www.sciencedaily.com/releases/2012/02/120216133442.htm Now, we can argue whether viruses are alive or not, but if there is a piece of DNA in the form of cancer that is spreading from animal to animal, should we consider the cancer to be alive or not? Elizabeth Murchison talks on TED about fighting this cancer and what the implications of this new cancer might be: http://www.ted.com/talks/elizabeth_murchison.html
As Dirk has pointed out, viruses are among the smallest microorganisms but the large majority of scientists consider them as just some nucleic acid (either RNA or DNA) with a protein coat and therfore not ‘living’!